The present invention relates to a probe for sensing a physical quantity.
Probes are commonly used in various branches of industry. They are usually installed in a container and are designed to sense a physical quantity.
Probes are for example employed for level measurements in industrial applications. To determine whether a medium in a container has reached or risen above a predetermined level a probe is inserted into the container such that its tip reaches the predetermined level. The tip is electrically conductive. If the container is electrically conductive a single probe is sufficient for the measurement. A conductivity between the container and the probe can be monitored with the probe. This conductivity changes dramatically when the level reaches the tip of the probe. In case of a non-conductive container a reference probe can be inserted next to the probe and the conductivity between the probe and the reference probe is an indicator for whether the level inside the container has reached the predetermined level.
Alternatively a radio frequency can be applied and a capacitance between the probe and the container or the reference probe can be evaluated.
Frequently more than one predetermined level need to be monitored. Correspondingly more than one probe can be inserted such that for every predetermined level there is a probe inserted having its tip positioned a that predetermined level. In addition to these measurement probes, again a reference probe can be inserted if necessary, as described above.
Frequently probes are also used for continuous level measurement. To this extend a probe extends into the container as far down as a level of a medium inside it shall be measured. There are several methods for continuous level measurement. For example a radio frequency can be applied to the probe and a capacitance between the probe and the container can be evaluated.
Another method consists in sending a short high frequency electromagnetic wave pulse down the probe. The pulse will be reflected at a surface of the level to be measured. Thus a time of flight for the pulse to travel down the probe and its reflection to return can be measured, to obtain a measure for the level inside the container.
Depending on the container and the height of the predetermined level, probes need to be supplied in various length. In industry, for example in the food and beverage industry vessel with heights of up to 30 m are used.
Existing designs use stainless steel rods as probes. Usually a process connection is foreseen, for mounting the probes on site.
In some applications bare metal rods are used and an insulating sleeve, for example out of polytetraflourethylen (PTFA) or perfluoroalkoxy (PFA), is foreseen to electrically insolate the rod against the process connection.
Other probes comprise a tight sheath of plastic, for example polytetraflourethylen (PTFA) or perfluoroalkoxy (PFA), that is molded onto the rod. The rod is cut to length and the plastic is trimmed back on one end to allow the stainless steel tip of the rod to be in electrical contact with the medium in the container.
A potential problem of the existing designs can occur, when the medium enters between the sleeve and the stainless steel rod or between the sheath and the stainless steel rod. This medium will stay there since industrial cleaning methods, in particular hygienic cleaning in place (CIP-Cleaning), are generally unable to reach between the sleeve and the rod or between the sheath and the rod respectively.
Especially in the Pharmaceutical Industry and the Food and Beverage Industry where hygiene standards are very high this can cause severe contamination problems.